1. Field of the Invention
This invention relates to catalytic reforming apparatus and processes, particularly for producing gasoline of reduced benzene content.
2. Description of Related Art
Refinery products used for fuels are receiving increasing levels of attention. Product specifications are being scrutinized by governmental agencies whose interests are decreased emissions from mobile and stationary sources, and by the industries that produce the engines and vehicles that utilize these fuels.
Regional and national regulations have been in place and continue to evolve concerning gasoline specifications, and automakers have proposed a set of limitations for gasoline and diesel to allow them to manufacture vehicles which will produce significantly lower emissions over their lifetime. Maximum sulfur, aromatics, and benzene levels of 10 parts per million by weight (ppmw), 25 volume % (V %), and 1 V % or less, respectively, have been targeted as goals by regulators. There are cost-effective technology options available to refiners that are offered by several commercial licensors to produce fuels meeting the specifications set for the gasoline pool.
Historically, lead was commonly added to gasoline to increase octane. When the use of lead was phased out due to environmental concerns, no direct substitute existed, and refiners instead have converted certain hydrocarbon molecules used in gasoline blending in order to achieve higher octane ratings. Catalytic reforming, which involves a variety of reactions in the presence of one or more catalysts in the presence of recycle and make-up hydrogen, is a widely used process for refining hydrocarbon mixtures to increase the yield of higher octane gasoline.
Although benzene yields can be as high as 30 V % in reformates, no more than 1 V % can be present in typical gasoline pools. There currently exist methods to remove benzene from reformate, including separation processes and hydrogenation reaction processes. In separation processes, benzene is typically extracted with a solvent and then separated from the solvent in a membrane separation unit or other suitable unit operation. In hydrogenation reaction processes, the reformate is divided into fractions to concentrate the benzene and one or more benzene-rich fractions are hydrogenated.
In a typical refinery, naphtha is reformed after hydrodesulfurization to increase the octane number of the gasoline. The reformate contains a high level of benzene which must be reduced in order to meet requisite fuel specifications that are commonly in the range of from about 1-3 V % benzene, with certain geographic regions targeting a benzene content of less than 1 V %. Benzene hydrogenation is a well-established process that can be used to reduce the benzene content of the reformate product stream.
A flow diagram including a prior art catalytic reforming process and apparatus 100 is shown in FIG. 1. A reforming unit 120 is integrated with a benzene saturation unit 130 for the processing of a hydrocarbon fraction to produce gasoline and light reformate. A naphtha stream 101 is first hydrotreated in hydrotreating unit 110 to produce a hydrotreated naphtha stream 102. Hydrotreating unit 110 operates under conditions of, e.g., temperature, pressure, hydrogen partial pressure, liquid hourly space velocity (LHSV), catalyst selection/loading that are effective to remove at least enough sulfur and nitrogen to meet requisite product specifications. For instance, hydrotreating in conventional naphtha reforming systems generally occurs under relatively mild conditions that are effective to remove sulfur and nitrogen to less than 0.5 ppmw levels.
The hydrotreated naphtha stream 102 is reformed in reforming unit 120 to produce a gasoline reformate product stream 103. In general, the operating conditions for reforming unit 120 include a temperature in the range of from 260° C. to 560° C., and in certain embodiments from 450° C. to 560° C.; a pressure in the range of from 1 bar to 50 bars, and in certain embodiments from 1 bar to 20 bars; and a LHSV in the range of from 0.5 h−1 to 40 h−1, and in certain embodiments from 0.5 h−1 to 2 h−1.
The total reformate stream 103 is passed to a reformate splitter 125 and separated into one or more relatively benzene-rich fractions 107 and one or more relatively benzene-lean fractions 104 and 106. Typically, a relatively benzene-rich middle fraction 107 comprises about 10-20 V % of the total reformate and contains about 20-30 V % benzene. In contrast, the relatively benzene-lean heavy reformate bottom fraction 106 comprises about 40-80 V % of the total reformate and has a benzene content generally in the range of from about 0.3-1 V %, which is sufficiently low to be passed to a gasoline pool 135 without further processing. The light reformate top fraction 104 which includes about 10-25 V % of the total reformate, contains about 5-30 V % benzene and is recovered or blended with other product pools.
The middle fraction 107, or “heart cut,” which contains a majority of the benzene content of reformate 103, is passed to a hydrogenation unit 130, which is also referred to as a benzene saturation unit, with a predetermined amount of hydrogen gas 105 for conversion reactions including conversion of benzene to cyclohexane and for the production of a benzene-lean and in certain embodiments an essentially benzene-free, gasoline blending component 108. The benzene saturation unit 130 typically contains an effective quantity of catalyst having a suitable level of active materials possessing hydrogenation functionality, such as nickel, platinum or other Group VIIIB metals, supported on an alumina substrate.
In general, the operating conditions for hydrogenation unit 130 include a temperature in the range of from 35° C. to 200° C., in certain embodiments from 95° C. to 140° C.; a pressure in the range of from 5 bars to 50 bars, in certain embodiments from 5 bars to 25 bars; and a LHSV in the range of from 0.5 h−1 to 10 h−1, in certain embodiments from 1 h−1 to 4 h−1.
The benzene-lean blending component 108 is mixed with the remaining gasoline pool constituents including the benzene-lean heavy reformate bottom fraction 106. For instance, when blended with the heavy reformate fraction 104 which can contain up to 1 V % benzene, a final gasoline product can be recovered which contains less than about 1 V % benzene.
A typical gasoline blending pool includes C4 and heavier hydrocarbons having boiling points of less than about 205° C. In the catalytic reforming process, paraffins and naphthenes are restructured to produce isomerized paraffins and aromatics of relatively higher octane numbers. The catalytic reforming converts low octane n-paraffins to i-paraffins and naphthenes. Naphthenes are converted to higher octane aromatics. The aromatics are left essentially unchanged or some may be hydrogenated to form naphthenes due to reverse reactions taking place in the presence of hydrogen.
The reactions involved in catalytic reforming are commonly grouped into the four categories of cracking, dehydrocyclization, dehydrogenation and isomerization. A particular hydrocarbon/naphtha feed molecule may undergo more than one category of reaction and/or may form more than one product.
Catalytic reforming processes are catalyzed by either mono-functional or bi-functional reforming catalyst which contains precious metals, i.e., Group VIIIB metals, as active components. A bi-functional catalyst has both metal sites and acidic sites. Refineries generally use a platinum catalyst or platinum alloy supported on alumina as the reforming catalyst.
The hydrocarbon/naphtha feed composition, the impurities present therein, and the desired products will determine such process parameters as choice of catalyst(s), process type, and the like. Types of chemical reactions can be targeted by a selection of catalyst or operating conditions known to those of ordinary skill in the art to influence both the yield and selectivity of conversion of paraffinic and naphthenic hydrocarbon precursors to particular aromatic hydrocarbon structures.
Referring to FIG. 2, a prior art process flow of an embodiment of a catalytic reforming system 200 is illustrated. Catalytic reforming processes typically include a series of reactors 260A, 260B, 260C and 260D which operate at temperatures of about 480° C. A feedstock 251 is introduced into a heat exchanger 250 to increase its temperature. The heated feedstock 252 is treated in the reforming reactors to produce a hot product hydrogen and reformate stream 261.
The reforming reactions are endothermic resulting in the cooling of reactants and products, requiring heating of effluent, typically by direct-fired furnaces 255A, 255B, 255C and 255D, prior to charging as feed to a subsequent reforming reactor. As a result of the very high reaction temperatures, catalyst particles are deactivated by the formation of coke on the catalyst which reduces the available surface area and active sites for contacting the reactants.
Hot product hydrogen and reformate stream 261 passes through heat exchanger 250 and then to separator 270 for recovery of hydrogen stream 271 and a separator bottoms stream 273. Recovered hydrogen stream 271 is split and a portion of the hydrogen 272 is fed to compressor 290 and recycled back to the reformer reactors with hydrogen stream 251. The remaining portion 274 of the hydrogen gas is sent to other refining unit operations, such as hydrotreating. The separator bottoms stream 273 is sent to a stabilizer column 280 to produce a light end stream 281 and a reformate stream 282.
There are several types of catalytic reforming process configurations which differ in the manner in which they regenerate the reforming catalyst to remove the coke formed in the reactors. Catalyst regeneration, which involves combusting the detrimental coke in the presence of oxygen, includes a semi-regenerative process, cyclic regeneration and continuous regeneration. Semi-regeneration is the simplest configuration, and the entire unit, including all reactors in the series is shutdown for catalyst regeneration in all reactors. Cyclic configurations utilize an additional “swing” reactor to permit one reactor at a time to be taken off-line for regeneration while the others remain in service. Continuous catalyst regeneration configurations, which are the most complex, provide for essentially uninterrupted operation by catalyst removal, regeneration and replacement. While continuous catalyst regeneration configurations include the ability to increase the severity of the operating conditions due to higher catalyst activity, the associated capital investment is necessarily higher.
The problem faced by refineries is how to most economically reduce the benzene content in the reformate products sent to the gasoline pool by modifying the processes and apparatus of existing systems practicing the prior art processes described above.